This applications claims priority from Japanese Patent Application No. 11-285632, filed Oct. 6, 1999.
The present invention relates to an adhesive, particularly a tacky acrylic-based heat sensitive adhesive. The invention further relates to, for example, retroreflective articles that employ the adhesive.
Indicators such as outdoor road signs, road markings and signboards have conventionally been equipped with visible retroreflective elements to provide surveyors with visible guidance. Retroreflective elements are usually worked into sheet forms and mounted on based materials such as aluminum sheets to form the indicators. When such indicators are situated on guard rails around corner turns, they can provide particularly effective visible guidance to drivers. In such cases, the indicator will usually be constructed of a retroreflective element on the surface of a curved base material.
Acrylic-based adhesives, which have excellent weather resistance, are typically used for mounting of retroreflective elements on base materials. Acrylic-based adhesives include types that are heat sensitive. Such heat sensitive adhesives are known as hot-melt adhesives, and they are heat activatable. That is, the heat sensitive adhesive may be heated to wet the adherend surface, and then the heat removed for hardening to complete the adhesive. No flammable solvent is required.
Acrylic-based heat sensitive adhesives are often given special properties by modification with various additives such as disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 61-9479, Japanese Unexamined Patent Publication (Kokai) No. 2-218780 and Japanese Unexamined Patent Publication (Kokai) No. 2-218779.
Japanese Unexamined Patent Publication (Kokai) No. 61-9479 discloses an acrylic-based heat sensitive adhesive to which a crystalline plasticizer is added. Addition of a plasticizer generally tends to lower the cohesive power of the adhesive. Typical retroreflective elements are protected by being covered with flexible plastic, and are sometimes formed on the surfaces of curved base materials as mentioned above. In such cases the retroreflective element deforms, producing bending stress. Therefore, when the bending stress lowers the cohesive power of the heat sensitive adhesive as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-9479, there is a tendency toward interlayer peeling, such as pop-off, to occur between the retroreflective element and the base material.
Japanese Unexamined Patent Publication (Kokai) No. 2-218780 discloses a heat sensitive adhesive designed to improve the flow properties on the adherend surface with heat, by addition of a benzylidene sorbitol derivative. However, the improvement in flow properties generally tends to reduce the cohesive power of the adhesive. This also therefore results in a risk of the interlayer peeling described above.
Japanese Unexamined Patent Publication (Kokai) No. 2-218779 discloses a heat sensitive adhesive designed to improve the cohesive power by addition of an N-acylamino acid amide. However, it is known that the N-acylamino acid amide is not able to impart cohesive power of a degree that can inhibit the aforementioned interlayer peeling for heat sensitive adhesives.
It is preferred for a heat sensitive adhesive to be given appropriate tackiness so that the retroreflective element can be slid on the substrate to alter its position, but can no longer be slid once fixed. In the heat sensitive adhesives disclosed in Japanese Unexamined Patent Publications (Kokai) No. 2-218779 and No. 2-218780, a tackifier is added as necessary to allow positioning of the retroreflective element, but for heat sensitive adhesives such as also disclosed in Japanese Patent Specification No. 2599725, this tends to cause a reduction in cohesive power and deterioration of the aging resistance. As a result, such heat sensitive adhesives do not allow fixing of the retroreflective element without sliding.
It is therefore an object of the present invention to provide an adhesive which exhibits high flow properties upon heating and the tackiness necessary to allow positioning by sliding, and which can exhibit cohesive power that prevents sliding and interlayer peeling after fixing, as well as a retroreflective member that employs it.
The present invention, in one embodiment, relates to an adhesive which is a reaction product of a monomer component containing 80-85 wt % of an alkyl (meth)acrylate ester monomer with an alkyl group of 1-18 carbon atoms and 15-20 wt % of an unsaturated monomer with at least one polar functional group, and which has a gel fraction of 50-85% after curing or crosslinking.